In general, electronic devices typified by semiconductors, and pharmaceutical products, and the like are manufactured in clean rooms. Such electronic devices, pharmaceutical products, and the like are affected in performance and quality by adhesion of fine particles and microorganisms in the process of manufacture. It is thus preferred that the clean rooms are kept in clean and sterile as much as possible.
Therefore, there have been conventionally used procedures for monitoring the clean condition of a clean room by measuring particles floating in the clean room with a particle counter.
As for measurement of particles floating in a clean room, there has been known a first prior art using an airborne particle counter (see, for example, JP-T-2009-539084: Patent Literature 1). The first prior art is characterized in that an airborne particle counter is equipped with a scroll pump and a photomultiplier-tube. This makes it possible to measure the number of particles in the air at the maximum flow rate of 100 L/min.
There also has been known a second prior art for collecting particles in the air by a collector prior to measurement of the particles by a particle counter (see, for example, Japanese Patent No. 4571623: Patent Literature 2). According to this prior art, a liquid is poured into the container of the collector, the air is sucked into the collector (container), and particles contained in the sucked air are introduced into the liquid by centrifugation.
According to the second prior art, the air can be sucked within the range of flow rates of 200 to 400 L/min., and besides particles in the sucked air can be collected into a liquid of 10 mL.
Besides the second prior art, there has been known a third prior art for collecting airborne particles in a liquid and measuring the number of particles (see, for example, Japanese Patent No. 3745719: Patent Literature 3). According to this prior art, water is reserved in a microorganism analyzer, and the (sucked) air is passed through the water to mix microorganisms and fine particles in the air with the water. Then, the water mixed with the microorganisms and fine particles is irradiated with a laser beam to measure the microorganisms with utilizing their autofluorescence phenomenon.
In recent years, there has been demand for a measurement technique as described below from the viewpoint of more efficiently monitoring the clean condition of a clean room. Specifically, there has been demand for a technique of collecting samples (air) as many as possible in a short time and measuring the number of particles contained in the collected samples in real time. In the first to third prior arts described above, it is simulated that particle measurement is performed at appropriate times at a place as a target of measurement. Accordingly, these prior arts are not suitable for use in automatic measurement of particles at a measurement place for a long period of time.
From the viewpoint of collecting and measuring a large number of samples, the first to third prior arts described above have problems described below.
Specifically, in the first prior art for an airborne particle counter, the more the measurable flow rate of air is increased, the longer the measurement time is taken. In addition, with an increase in flow rate of the air, the inner diameter of a flow cell becomes larger corresponding to the flow rate. This requires expansion of a region for detection of particles. Thus, the range of laser irradiation needs to be expanded. In this case, expanding the range of laser irradiation leads to decrease in the energy density of laser beam. This causes a problem of deteriorated detection sensitivity.
As a countermeasure for the above problems, maintaining the energy density of the laser beam is necessary in order to maintain the sensitivity of detection by increasing the flow rate of the air as well as the flow velocity in the flow cell. In this case, a high-power laser beam source corresponding to the range of irradiation mentioned above is needed. Alternatively, a high-sensitivity light receiving element is needed. However, the use of the high-power laser beam source or the high-sensitivity light receiving element results in cost increase.
As described above, it is extremely difficult to use an airborne particle counter to collect a large number of samples and maintain high detection sensitivity for the particle diameter. If an attempt is made to harmonize collection of a large number of samples with maintenance of high detection sensitivity, the degree of technical difficulty becomes high. Therefore, the first prior art is not suitable for use in collecting a large number of samples and automatically measuring the number of particles at a measurement place for a long period of time.
Meanwhile, according to the method for measuring the number of particles collected in a liquid, the amount (flow rate) of air sucked can be raised without increasing the amount of the liquid as a sample.
The second prior art is thus suited to collecting a large number of samples. However, on measurement of samples with the particle counter, the operator needs to remove the container as a collector and set the container in the particle counter. Thus, the second prior art is not suitable for use in so-called constant monitoring by which changes in the number of particles are monitored for a long time (for example, 24 hours).
According to the third prior art, the state of the air in batches can be measured at a certain time. However, the third prior art does not allow measurement of chronological changes in the number of particles. Therefore, the third prior art is not suitable for use in constant monitoring as with the second prior art.
Under such circumstances as described above, there has been demand for a technique suitable for monitoring of clean condition in relation to collection of particles contained in the air and measurement of the number of the particles.